Temperature compensated snap-beam actuator

ABSTRACT

A snap acting thermal actuator apparatus and more particularly a temperature compensated snap beam actuator, one embodiment disclosing a snap beam thermal sequencer. The thermal electric snap beam actuator apparatus is operable to either of two retained positions dependent on the temperature of the thermal snap beam element. The bimetal snap beam element is ambient temperature compensated by another bimetal beam. The end loading of the compensator beam is less than that of the snap beam, the compensator beam end loading preferably being barely equal to the buckling force of the beam, at which point the compensator beam presents a constant force at all displacement positions within the operating range and thus does not load down the snap beam. The compensated snap beam is also disclosed in multiple in a sequencer switch arrangement. The plurality of snap beams are heated together and by biasing the snap beams with end loads differing from each other they will be caused to operate at different operating points, such as in sequential action.

United States Patent n 1 Bassett TEMPERATURE COMPENSATED SNAP-BEAM ACTUATOR [75] Inventor: William W. Bassett, Wayzata, Minn.

[73] Assignee: Honeywell Inc., Minneapolis, Minn.

[22] Filed: Nov. 21, 1973 [21] Appl. No.: 417,862

Primary Examiner-Harold Broome Attorney, Agent, or Firm-Omund R. Dahle men EXPANSION SIDE END LOAD END LOAD PCRIT.

[ Sept. 3, 1974 [57] ABSTRACT A snap acting thermal actuator apparatus and more particularly a temperature compensated snap beam actuator, one embodiment disclosing a snap beam thermal sequencer. The thermal electric snap beam actuator apparatus is operable to either of two retained positions dependent on the temperature of the thermal snap beam element. The bimetal snap beam element is ambient temperature compensated by another bimetal beam. The end loading of the compensator beam is less than that of the snap beam, the compensator beam end loading preferably being barely equal to the buckling force of the beam, at which point the compensator beam presents a constant force at all displacement positions within the operating range and thus does not load down the snap beam. The compensated snap beam is also disclosed in multiple in a sequencer switch arrangement. The plurality of snap beams are heated together and by biasing the snap beams with end loads differing from each other they will be caused to operate at different operating points, such as in sequential action.

17 Claims, 6 Drawing Figures COMPENSATI'NG BEAM l l l l VA ll DRIVER SNAP BEAM PAIENIEDSEP awn 3.883.878

' U 1 UP 3 HIGH EXPANSION coMPENsATING SIDE BEAM I20 END LOAD A r CRIT. IPFORCE 2| Is m- 6 I; Z I ls a l8 END LOAD DRIVER SNAP" cRIT. BEAM FIG. I

PosIT vE GRADIENT 5, v INcREAsING END FORCE 6; I CAUSES GRADIENT TO Ca V CHANGE FROM P08. To NEG. 5 c

ZERO GRADIE T \B I! N LLI U 5 u. A

NEGATIVE GRADIENT I COLD STOP- J WARM sToP DISPLACEMENT FIG.2

PATENTEDSEP 19 4 sum 30F 3 FIG.6

TEMPERATURE COMPENSATED SNAP-BEAM ACTUATOR BACKGROUND AND SUMMARY OF THE INVENTION The general utility of snap-beam actuators has been well documented in the prior art thermal switches such as the expired U.S. Pat. No. 2,417,912 which teaches a temperature responsive bimetal beam in which the ends are constrained and the central portion is free to move, the beam being end loaded to provide a snap action of the beam. The use of two beams connected reverse or opposite to one another to provide temperature compensation is taught in the expired U.S. Pat. No. 2,002,467 entitled Snap Acting Thermal Device. Both of the above references also teach adjustable means for controlling the end loading of the snap beams.

The provision of a temperature compensating beam in addition to the snap beam has been a desirable feature, however, it has had one undesirable aspect in that it typically requires oversizing the driver bimetal snap beam since part of the energy will be absorbed by the compensator beam. The specific beam arrangement described herein covers an embodiment of temperature compensation which requires no additional power from the driver by providing a critical compressive longitudinal loading on the compensator bimetal beam.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagrammatic representation of the compensated snap beam of this invention;

FIG. 2 is a graphical representation of the effects of end loading of the bimetal beam;

FIG. 3 is a top plan view of a sequencer using a multiple number of the compensated snap beams of FIG. 1;

FIG. 4 is a pictorial or perspective view of the heating oven portion of the sequencer;

FIG. 5 shows more details of a stop plate which is also shown in FIGS. 3 and 4; and, FIG. 6 shows a preferred form of the bimetal beams of this invention.

DESCRIPTION Referring now to FIG. 1 there is shown a compensated snap beam assembly comprising a snap beam 10 and a compensating beam 11 which are mechanically linked at a midpoint of each beam by an actuating arm 12. The beams 10 and 11, which may be identical to one another if desired, are conventional thermally responsive bimetallic strips, and may be constructed as shown in FIG. 6. In FIG. 6, the beams are shown as having a normal curvature at ambient room temperature for a purpose to be described later. The beam need not have the curvature. The ends of the beams are fixed in position in the assembly, the central portion being free to move between stops. The right hand ends of beams 10 and 11 seat in V-notches I3 and 14 of a support or frame 15. Compression force or end loading force is applied to the opposite end of the snap beam 10 and also of the compensating beam 11 by loading means which here are shown as resilient means or springs 16 and 17, respectively. The compressive loading force on each beam can be individually adjusted by adjusting screws 20 and 21 which are threaded into frame member 15'. The screw, spring and beam are longitudinally or axially aligned. A heating means for snap beam 10 is herein shown as an electrical heating coil 18 around the beam which is energized. when it is desired that beam 10 take the warm position 10'.

Ambient temperature compensation is regularly achieved in the prior art by using two bimetal elements back-to-back, i.e., oppositely arranged, such that the net force change is zero so long as both elements see the same ambient temperature. This means of temperature compensation is effective but typically requires oversizing the driver bimetal since part of the driver bimetal energy is taken by the compensator because of lost motion or by compressing a spring member, i.e., the compensator. The specific beam arrangement described herein covers a novel bimetal temperature compensation which requires no additional power from the driver.

The operation of the snap beam is largely dependent on the end load applied to the snap beam. Three basic conditions can be established based on the amount of end loading. Referring now to the graph of FIG. 2 there are shown curves of force vs. displacement of bimetal beams under several end load conditions. In considering this graph the critical end load, which is the buckling load, is defined as that end load which when applied along the axis of the beam will barely cause buckling of the beam. Graph line A shows force vs. displacement when the end load to the beam is less than the critical or buckling load. Under this condition the beam has a positive spring gradient, as a force must be applied in order to move the beam. Graph line B describes the condition when the end loading of the beam is equal to the critical load. Under this condition the beam will have zero or no spring gradient since no additional force is required to move the beam. Graph line C shows the force vs. displacement when the end load exceeds the critical load. The beam will be in a buckled position and it is necessary to apply an initial force which will be stored until such time as this energy is released. The beam then snaps through center providing energy as it moves through a displacement. In this region the beam has a negative spring gradient characteristic. The gradient characteristic in each case is determined by the end load on the beam.

The snap beam 10 is loaded with an end load exceeding the buckling load and operates in the manner of Graph line C. The two stable positions of the beam 10 are shown in FIG. 1, the solid line 10 showing the cool (absence of heating) temperature position of the bimetal beam and the dashed line 10' showing the warm temperature position of the beam, the beam moving suddenly or snapping from one position to the other as the temperature of the beam 10 warrants. Since compensator beam 11 is mechanically linked to snap beam 10 by arm 12, the two beams move together as one. In other words, the thermally responsive beam 10, having its ends restrained, has a central portion movable in a first direction upon heating and in a second direction upon cooling. The beam 10 by sufficient end loading is provided with a snap action from one to the other of two positions, one on either side of center. The thermally responsive beam 10 may if desired, be made to have a curvature at ambient room temperatures (i.e., the cool temperature), as shown in FIG. 6, which curvature aids in returning the beam against the cool stop of stop plate 44 except during the time the beam is heated by the electrical heating means 18 or 35 and snaps over to the warm position against the warm stop.

The compensating beam 11 is preferably loaded with an end load equal to the buckling load, as shown in Graph line B. When the end force on beam 11 is equal to the buckling load, the force provided by the compensating beam will not be influenced by its position at 11 or 11 or inbetween since the beam has a zero gradient. Beam 11 thus does not provide a loading effect on snap beam 10'. A change in temperature, however, will cause the force to change since the bimetal beam 11 produces a force based on its internal moments. The force produced by the compensator 11 is therefore responsive to temperature changes, but is independent of position. The significance of this compensator is that it provides temperature compensation without absorbing any energy from the driving member, that is from snap beam 10, when it is displaced. The snap beam 10 thus does not have to be oversized to provide the desired work output. The temperature of the beam 10 at which it snaps from the cool position to the warm position 10 is controllable by changing the set screw 20 to increase or decrease the end force. By adjusting screw 21 the end force on compensating beam 11 can be adjusted slightly above or slightly below the critical force if desired to provide further calibration.

SEQUENCER The compensated snap beam of FIG. 1 is utilized in multiple in an electric sequencer apparatus shown in FIG. 3. It is desired to have apparatus for the switching of a plurality of electrical loads in which there is a time interval between the switching of the several loads. In the apparatus of FIG. 3 there may be four compensated snap beams to provide a time interval between the switching of each of four electrical loads. This precise number of sequencing switches is not significant and is illustrative only. In the center of FIG. 3 is a heating chamber or oven having within mounted a plurality of snap beams of which 10 and 1 10 are shown. Two additional sets of beams may be located immediately below the shown members. FIG. 4, which is a pictorial view of the oven 30, may be considered together with FIG. 3 during the description of the oven and the snap beams contained therein.

The-oven assembly comprises slotted end members 31 and 32 and top and bottom longitudinal connecting members 33 and 34. The heating coil 35, a few turns of which are shown in FIG. 3, is wound around the members 33 and 34 to form an oven within for heating the snap beams when the coil terminals are energized. In the end member 31 are a plurality of notches l3, 14, 113 and 114 to receive the ends of the beams. The beams may be vertically retained in position by the small slots in the V-notch area into which the tab end 41 of the beam is inserted. The other end member 32 has wider slots 42 through which the snap beams may pass and extend, and similar slots 43 through which the compensating beams may extend. In the center of the oven assembly is a stop plate 44 which controls the amount of movement of the snap beam between the cold position and the warm position. Stop plate 44 is shown in its entirety in FIG. 5, and the slot areas 45 and 46 through which the snap beams pass are indicated as being the same for all of the several snap beams. The slot areas need not be the same for each snap beam and may be modified as desired, and two sample modifications as suggested by dashed lines 47 and 48 show how the cold stop can be changed at 47 and how the warm stop can be changed at 48. The operational graph of FIG. 2 indicates the operational changes as these stops are changed.

The springs 16, 116, 17 and 117, as shown in FIG. 3, which provide the end loading for the snap beams 10 and and the compensating beams 11 and 11 may have adjusting screws as described in FIG. 1 or the compensating springs 17 and 117 may be of a strength to apply the critical end load to the compensating beams 11 and 111, while the snap beam springs 16 and 116 apply an end force greater than the critical buckling force to the snap beams 10 and 110. The snap beam springs 16, 116 etc. may be identical one to the other or may have a spring force which is slightly different one from the other. If snap beam springs are chosen which are slightly different, then the graph line C of FIG. 2 rotates slightly as at C for example, as each such spring loads its associated snap beam, by a slightly different amount. As the differently loaded snap beams are simultaneously heated, the switching of the beams occurs sequentially rather than all at one time. This is because different end loadings cause a change in operating point. Thus if the end loading on a beam is increased, the force F at which switching takes place is increased, meaning that the beam must be heated more than before. The discussion to this point has been on the basis that all of the beams, snap beams and compensator beams are identical. This need not be so, however, the beams can be characterized in dimension or otherwise or the slots of the stop plate may be characterized to accomplish the same result as the varied end loading of identical beams.

The switches 12a and 112a of FIG. 3 have been shown as conventional normally closed SPST leaf switches, however, it is obvious that any suitable switch can be used.

The embodiments of the invention in which an exclusive property or right is claimed are defined as follows:

first loading means to provide a compressive loading throughout a first of said bimetal beams in excess of said critical value whereby said beam snaps from one to the other of two positions dependent upon the temperature of said beam;

second loading means to provide a compressive loading throughout the second of said bimetal beams to approximately said critical value at which loading said second beam can assume changes in displacement with no change in force whereby said second beam operates as a temperature compensator for said first snap beam without changing the loading on said first beam;

and electric heating means associated with said first snap beam for operating said first beam in said first direction when heated and thereby operating both of said beams from said one position to said other position.

2. The invention according to claim 1 and further comprising:

contact means operated in accordance with the position of said beams.

3. The invention according to claim 1 wherein said first and second loading means comprise resilient means.

4. The invention according to claim 1 and further comprising:

stop means positioned to constrain the movement of said beam.

5. A temperature compensated thermal actuator comprising:

a first and second thermally responsive beam, said beams having their end positions relatively fixed and having a central portion movable in a first direction upon heating and in a second direction when heating ceases, said first beam being provided with a snap action throughout a certain range of its movement;

first support means mechanically interconnecting and positioning one end of said first and second beams;

second support means interconnecting and positioning the other end of said first and second beams so that said beams are positioned relatively parallel to one another, said beams being oppositely arranged to provide temperature compensation;

first spring means associated with said support means and positioned to apply compressive end loading to said first beam, said loading exceeding in magnitude that necessary to cause buckling of said beam to cause said snap action;

second spring means associated with said support means and positioned to apply a lesser magnitude of compressive end loading to said second beam which loading is approximately that necessary to cause buckling of said second beam;

heating means associated with said first beam to actuate said first beam when heated;

mechanical link means connecting the central portions of said first and second beams so that they move as a unit;

and electrical contact means operated in accordance with the position of said beams.

6. The invention according to claim 5 in which said snap beams are made with an initial longitudinal curvature to cause return in said second direction upon cessation of heating.

7. The invention according to claim 5 and further comprising stop means positioned to constrain the movement of said beam within said snap acting range.

8. The invention according to claim 5 and further comprising:

adjustable means associated with at least one of said LII first and second beams for adjusting the magnitude of said compressive end loading.

9. The invention according to claim 8 wherein said adjusting means comprises screw means.

10. The invention according to claim 8 wherein said adjustable means, said spring means and said beam are longitudinally aligned.

11. A snap beam type thermal sequencer comprising:

a plurality of thermally responsive bimetal snap beams; loading means applying a compressive longitudinal loading on each of said plurality of thermally responsive snap beams, said means loading each said beam in excess of its buckling load so that said beams are biased to either of two positions, said snap beams having a central portion which snaps in a first direction upon heating and in a second direction when heating ceases and the beam cools;

electrical heating means positioned in heat transfer relation with said snap beams whereby said beams can be simultaneously heated, said means comprising an electrical heating chamber having first and second end member means and having longitudinal connecting member means extending between said end member means, the end member and connecting member means defining said chamber, said first and second end member means receiving and positioning said plurality of snap beams within said chamber, said chamber including power input terminals for connection to a controlled supply of power whereby said chamber can be heated to simultaneously heat said beams;

and means coupling the position of said snap beams to switching means.

12. The sequencer of claim 11 wherein each of said beams is compressively loaded to a slightly different value to cause a sequential operation of said beams.

13. The sequencer of claim 11 in which said loading means comprises individual spring means for end loading each of said beams.

14. The sequencer of claim 13 in which said individual spring means each applies a spring load to its respective beam which differs from the others.

15. The sequencer of claim 13 in which said individual spring means further comprise adjusting means to individually adjust the end loading applied to the beams.

16. The sequencer of claim 11 and further comprising:

a plurality of thermally responsive compensator beams, one for each of said snap beams;

said first and second end member means also being adapted for receiving and positioning said plurality of compensator beams such that one compensator beam is substantially parallel to each snap beam.

17. The sequencer of claim 11 and further comprising:

stop means positioned to constrain the movements of said beams to define said two positions of each of said beams. 

1. A temperature compensated thermal actuator comprising: two substantially parallel thermo responsive bimetal beams, one beam a thermal operator and the other a temperature compensator for the thermal operator oppositely arranged, each beam having a central portion movable in a first direction upon heating and in the opposite direction upon cooling; said beams being mechanically linked at a position intermediate said ends so that the beams move together as a unit, said beams being subject to longitudinal buckle upon the application of a compressive loading thereto which exceeds a critical value; first loading means to provide a compressive loading throughout a first of said bimetal beams in excess of said critical value whereby said beam snaps from one to the other of two positions dependent upon the temperature of said beam; second loading means to provide a compressive loading throughout the second of said bimetal beams to approximately said critical value at which loading said second beam can assume changes in displacement with no change in force whereby said second beam operates as a temperature compensator for said first snap beam without changing the loading on said first beam; and electric heating means associated with said first snap beam for operating said first beam in said first direction when heated and thereby operating both of said beams from said one position to said other position.
 2. The invention according to claim 1 and further comprising: contact means operated in accordance with the position of said beams.
 3. The invention according to claim 1 wherein said first and second loading means comprise resilient means.
 4. The invention according to claim 1 and further comprising: stop means positioned to constrain the movement of said beam.
 5. A temperature compensated thermal actuator comprising: a first and second thermally responsive beam, said beams having their end positions relatively fixed and having a central portion movable in a first direction upon heating and in a second direction when heating ceases, said first beam being provided with a snap action throughout a certain range of its movement; first support means mechanically interconnecting and positioning one end of said first and second beams; second support means interconnecting and posItioning the other end of said first and second beams so that said beams are positioned relatively parallel to one another, said beams being oppositely arranged to provide temperature compensation; first spring means associated with said support means and positioned to apply compressive end loading to said first beam, said loading exceeding in magnitude that necessary to cause buckling of said beam to cause said snap action; second spring means associated with said support means and positioned to apply a lesser magnitude of compressive end loading to said second beam which loading is approximately that necessary to cause buckling of said second beam; heating means associated with said first beam to actuate said first beam when heated; mechanical link means connecting the central portions of said first and second beams so that they move as a unit; and electrical contact means operated in accordance with the position of said beams.
 6. The invention according to claim 5 in which said snap beams are made with an initial longitudinal curvature to cause return in said second direction upon cessation of heating.
 7. The invention according to claim 5 and further comprising stop means positioned to constrain the movement of said beam within said snap acting range.
 8. The invention according to claim 5 and further comprising: adjustable means associated with at least one of said first and second beams for adjusting the magnitude of said compressive end loading.
 9. The invention according to claim 8 wherein said adjusting means comprises screw means.
 10. The invention according to claim 8 wherein said adjustable means, said spring means and said beam are longitudinally aligned.
 11. A snap beam type thermal sequencer comprising: a plurality of thermally responsive bimetal snap beams; loading means applying a compressive longitudinal loading on each of said plurality of thermally responsive snap beams, said means loading each said beam in excess of its buckling load so that said beams are biased to either of two positions, said snap beams having a central portion which snaps in a first direction upon heating and in a second direction when heating ceases and the beam cools; electrical heating means positioned in heat transfer relation with said snap beams whereby said beams can be simultaneously heated, said means comprising an electrical heating chamber having first and second end member means and having longitudinal connecting member means extending between said end member means, the end member and connecting member means defining said chamber, said first and second end member means receiving and positioning said plurality of snap beams within said chamber, said chamber including power input terminals for connection to a controlled supply of power whereby said chamber can be heated to simultaneously heat said beams; and means coupling the position of said snap beams to switching means.
 12. The sequencer of claim 11 wherein each of said beams is compressively loaded to a slightly different value to cause a sequential operation of said beams.
 13. The sequencer of claim 11 in which said loading means comprises individual spring means for end loading each of said beams.
 14. The sequencer of claim 13 in which said individual spring means each applies a spring load to its respective beam which differs from the others.
 15. The sequencer of claim 13 in which said individual spring means further comprise adjusting means to individually adjust the end loading applied to the beams.
 16. The sequencer of claim 11 and further comprising: a plurality of thermally responsive compensator beams, one for each of said snap beams; said first and second end member means also being adapted for receiving and positioning said plurality of compensator beams such that one compensator beam is substantially parallel to each snap beam.
 17. The sequencer of claim 11 and further comprising: stop means positioned to conStrain the movements of said beams to define said two positions of each of said beams. 